1. Field of the Invention
This invention relates to the field of infrared optical detectors, and in particular to optical waveguide end-coupled infrared detectors.
2. Description of the Related Art
Infrared detectors are key components for optical communication systems. They are responsible for performing the signal conversion from the optical to the electrical domain at the receiver end. An optical waveguide is an elongated structure having a 2-dimensional cross-sectional pattern, which guides light inside it like a water conduit carries water inside. Since modern optical signal processing is often realized by waveguide based devices in contrast to the traditional bulk optics, waveguide detectors are important components of such systems. As a receiver end device, waveguide end-coupled infrared detector is expected to reduce the receiver form factor, reduce cost and improve the receiver performance.
In an optical communication system, the infrared detectors are typically made of p-n junction based photodiodes (PDs). Traditional PDs are designed to be normal incident type. The PD's high speed performance is limited by the trade-off between speed and responsivity, where responsivity represents the conversion efficiency of light to electrical signal. If the light absorption layer of a PD is thicker, more light could be converted to the electrical signal under a static bias condition; however, photo-generated carriers have to travel a longer distance to reach the top and bottom p or n contact, which means a slower device. A waveguide based PD can offer a solution to this limitation by decoupling light absorption and carrier drifting paths. This is normally achieved by designing the photo-carrier collection electrical field inside the light absorption material to be perpendicular to the direction of waveguide, i.e. the direction of light propagation. However, in order to make the photo-detector fast enough, the lateral dimension, i.e. either the width or thickness of the waveguide needs to be small. For a modern 10 Gbps detector, this thickness is roughly limited to be about 1-2 μm. This smaller lateral dimension increases the difficulty of fiber coupling to the waveguide device.
Waveguide detectors, owing to their smaller cross-sectional dimension, are typically used in high speed (10 Gbps or higher), long wavelength (1250-1610 nm) infrared communications. These detectors typically comprise a light carrying medium and a light absorbing medium, most commonly, both in waveguide format. In these applications, light signal is typically carried in a single mode fiber (SMF). In a typical configuration, light from a SMF is end-coupled into a signal-carrying waveguide, from where it enters the light absorbing medium. When a fiber is connected to a waveguide, loss of light happens due to the mode mismatch between a single-mode-fiber and the optical mode inside a waveguide. If a waveguide detector is to perform efficient optical to electrical signal conversion, it needs to receive as much light from the fiber as possible and convert the optical to electrical signal at a high enough speed. As pointed out earlier, there is a trade-off between the SMF to waveguide connection and the device speed. A typical solution to this challenge is to design a waveguide with large lateral dimension on one end, and smaller lateral dimension on the opposite end, which is the end connecting to the light absorbing material. The large-lateral-dimension waveguide has a good mode-matching property to the SMF; the smaller lateral dimension end can facilitate a faster device. A mode convertor is needed to shape the light from one end with large mode cross-section, to the other end with a much smaller cross-section. Due to the limitation in silicon device processing, mode convertor is a difficult technology to implement.
Germanium is a material fully compatible to the standard complementary metal-oxide-semiconductor (CMOS) process and absorbs light up to 1600 nm optical communication wavelength range. High speed waveguide integrated Ge PDs have been extensively studied in the past decade. Most of the work is focused on sub-micron core or small core silicon-on-insulator (SOI) waveguides. However, inefficient coupling to SMF limits their practical deployment in real optical networks. Large core waveguides have the advantage of easy fiber coupling. So far, there are no demonstrated Ge waveguide PD in fiber matched mode size that can operate at 10 Gbps or higher speed owing to the long carrier drift time in large structures.